That is Brahman and the Bhagavad Gita describes the word as Tat. It is one of the names of the Brahman and is used by people who want to gain freedom from bondage. They say Tat, referring to the Brahman, and take up actions without attaching themselves to the results or identifying themselves as the doer. The Atma Bodha of Sri Adi Sankaracharya says Tat is Brahman.
How to arrive at Tat? This is an interesting exercise in Vedanta. Identify all that is Atat —not that—and negate them first. One after the other, everything that we see, hear, smell, taste, touch, experience, think, feel, plan, do or know gets knocked off in this process. If you can experience or think about something, someone or some situation—all of that comes under the category of Atat. Then what is Tat? Well, the one who negates everything and eventually comes to the realisation that the one who negates alone is Tat! The seer of all the actions, words, thoughts and feelings is Tat—the supreme truth.
Is it some strange practice that has been discovered by somebody that you tell us to follow? Not at all. This process of arriving at the truth of the self has been taken up by very great masters of Vedanta in the past ages. Their goal of life has been to negate each and everything that they encounter and experience and finally arrive at what they search for in their own self as the seer, knower and experience of all that is.
What is left out after this exercise? Advayam—the one without the second. The non-dual reality called Awareness alone remains. That alone is you and me, and everyone and everything else. There is nothing other than this awareness to know of or that can be known of. This is not some inert thing called consciousness. It is the very life of our being.
The very presence is always giving us joy. All the joy and happiness that we ever hope to experience is not because of the object of experience but due to the presence of this indivisible bliss in us called life. If it is not there, there is no one to experience bliss. To realise this is to reach the final single purpose of life. What is the constant refrain command of all the spiritual texts that bring us to the awareness of the self? Avadharayet—an imperative word. Concentrate. Focus. Meditate. Bear in mind. The only object of meditation is this Truth.